The more you look back on the 2012 season, the more ridiculous it is that the Indianapolis Colts[1] won 11 games and made the playoffs.

There’s almost no explaining it without resorting witchcraft as an explanation.

The Advanced Stat of the Week this week is Passer Rating Differential.

Passer rating has scores of critics. The most common complaints are that it overvalues completion percentage, it has no opponent adjustment, it has no way of adjusting for game situation, it doesn’t value running at all, it doesn’t account for sacks or fumbles, it is ridiculously hard to calculate and it was scaled in the 1970s, making it notoriously useless for comparing quarterbacks of different eras.

I realize that I’m being uncharacteristically kindhearted to the deeply flawed and possibly useless number, but passer rating is here to stay and is still infinitely more useful than ranking teams and players by yards. Passer rating is so painfully mainstream that addressing it as an “advanced stat” would make a mockery of the holy name of advanced stats.

Passer Rating Differential (PRD) essentially compares offensive and defensive passer ratings. One of the reasons I enjoy it so much is that the top eight teams in PRD all made the playoffs in 2011. It covers the gamut of teams from the Packers[3] and Saints[4] (great offensive passer ratings, mediocre defensive ones) to the Ravens[5] and 49ers[6], who posted outstanding defensive numbers.

PRD shows us that it doesn’t matter what combination you use to get the job done. Stopping the pass and passing the ball is the key to winning in the NFL[7].

What’s remarkable is how bad the Colts were at all of it.

Of course, they didn’t run the ball or stop the run very well either, so that theory is out the window too.

In 2011, the Colts had one of the worst PRDs in the NFL. They were at -31.7 on the year.

Despite cutting that gap in half, they were still one of the worst teams as judged by this metric, finishing 27th.

As predicted, Andrew Luck[8] helped, but in terms of passer rating, the effect wasn’t profound. Passer rating doesn’t capture everything Luck brought to the table, obviously, but his actual impact as far as the stat goes was slight. He boosted the Colts’ offensive number by just a shade over four points.

Defensively, the Colts actually jumped forward quite a bit, aided in part by one of the easiest possible schedules of opposing passers. The Colts had 10 games against the 13 worst quarterbacks in the league at passer rating.

In terms of raw productivity, Indy did a better job by limiting opponents to a completion percentage of 62.4. That’s nearly 10 percentage points better than in 2011 when they were last in the league allowing better than 71 percent of all passes to be completed.

Still, despite the improvements, the Colts were still among the worst teams in the league in PRD. Moving forward, they aren’t going to be able to replicate their playoff run without massive improvement on both sides of the ball.

Defensively, that will mean a complete overhaul of the secondary. The team has serious needs a safety and corner. Upgrading the lackluster pass rush would also help.

Overall, another similar five to 10-point improvement on defense would give the Colts a mediocre defense. Given that they’ll face a tougher slate of offenses, that would be an outstanding result.

Offensively, the change in coordinators to Pep Hamilton should result in a meteoric rise in Luck’s passer rating. Passer rating can be punitive to low-completion percentage offenses. That’s an inherent weakness in the stat itself, but without Bruce Arians’ bombs-away mentality, Luck should complete a higher percentage of his passes which alone should improve his metric, even if it doesn’t necessarily make the offense better.

Should Indy return to the postseason in 2013, their PRD will likely have to swing to the positive side.

Given the trajectory of the roster, that’s an entirely reasonable proposition.